Field of the Invention
The rolling resistance of one-ply tires is believed to be superior to that of multiple ply tires. Because tires with low rolling resistance conserve energy, a one-ply tire is a desired form of tire.
An increasing proportion of the tires which are being manufactured today are one-ply radial tires. When certain materials which have high modulus but poor resistance to compression fatigue, such as fiberglass, spun steel wire and aramid, are used as the one ply of a radial tire, there is a tendency for the ply to break in the crown area. These breaks can lead to breaks in the inner liner and tire failure. This is believed to be caused by compressive forces from the pantographing action of the cincture belt of the tire.
By high modulus is meant an initial modulus of from 200 to 600 GPD/100% or higher. GPD/100% means grams force per denier at 100% elongation as determined by ASTM Procedure D885-76.
Radials are now predominantly two-ply construction but there is a trend toward one-ply for reasons of economy and also lower rolling resistance. The one-ply radial can be made with nylon or polyester body cords because of the good resistance to compression fatigue but there are advantages in using high modulus cords. The high modulus materials generally have low thermal shrinkage which contributes to good dimensional stability and also uniformity. Unfortunately, poor resistance to compression and compression fatigue accompanies the high modulus properties.
The trend is toward these high modulus materials and the invention is a way of getting around the problems which will probably be encountered in one-ply high modulus plies. This problem exists with Kevlar (aramid), fiberglass and fine filament steel cords, the biggest problem being with glass.
Radial tires are ordinarily made on special equipment but can be and are made on conventional equipment. We prefer to use radial tire equipment, however.
The problem of ply cord breakage in the crown of the tire probably would not occur in some prior art configurations. Examples of prior art where ply cord breakage would not occur in the crown area are those described by Bourdon in U.S. Pat. Nos. 2,493,614 (1950) and 3,192,984 (1965). What Bourdon does is to eliminate the center cord plies and replace them with a high modulus rubber. According to Bourdon this provides a softer ride and prevents, to some degree, noise transmission that would normally occur throughout the tire cords. A variation of the Bourdon theme is found in Australian Patent Specification No. 29,833/71 Guyot.
The Guyot patent specification is directed to a tire having carcass ply except in the crown area. In the crown area Guyot eliminates the carcass ply.
Some of the problems encountered with the prior art tires are as follows. The carcass of a radial tire is normally manufactured on a flat drum. It would appear that it would be easier for a tire builder to apply a single radial cord ply to the drum and center it rather than apply two relatively narrow partial plies, one on each side of the tire, and get them absolutely parallel. It would seem that force variations might exist around the circumference of a tire during use if the partial plies were not aligned properly.
Another problem which would occur is: during tire building, the tire carcass, as it is made on a flat drum, is expanded to a toroidal shape. If there was no carcass ply in the center of the tire but only an insert under the ply and the tire were expanded with an inflatable rubber bladder, the center of the carcass would expand as the cord ply would be pulled from the center due to cords sliding in the rubber. This is particularly true of the newer glass cords which have low cohesiveness to uncured rubber compound. The prior art tires discussed here are the Bourdon and Guyot tires.
Another problem with the Bourdon tires which eliminates carcass ply from the center of the tire is that of crack propagation. Bourdon replaces the strength of tire cords with high modulus rubber. When a crack forms in such a material, continued flexing of the tire will cause the crack to propagate through the rubber.